The invention relates as a whole to composite structures and methods of forming thereof and can be used in particular, in the manufacture of bodies and compartments of flying vehicles used in rocketry and aeronautics.
Stringent requirements are imposed upon composite shells shaped as a body of rotation in what concerns their air-tightness, strength and reliability, and these products must therewith possess minimum weight. However, in solving the problems of air-tightness with a simultaneous reduction in weight, there arises a problem of premature destruction of products. Therefore, various sealing interlayers are included in the composite shells shaped as a body of rotation.
A composite shell is designed so as to take into account that a binder in a composite material of the product acts as a medium uniformly transmitting stresses in all directions, the maximum allowable strains of the binder being higher than those of fibers. Utilization of a suitable binder which has a substantially lower modulus of elasticity than the fibers ensures that a fiber will be destructed, not the binder, thus maximum efficiency will be provided for the selected structure (D. V. Rosato, K. S. Grove, Glass Fiber Winding, Moscow, Mashinostroienie Publishing House, 1969, pp. 188-191, 207-220).
In order to improve binding of fibers with the matrix-binder, they are impregnated with a latex compound (V. V. Ragulin "Technology of Tire Production", 2-nd Ed., Moscow, Khimia Publishing House, 1975, pp. 93, 96).
However, these solutions are not sufficient, fibers are destructed prematurely.
As shown by tests, destruction of a composite is influenced by differences in fiber orientation and by tangential stresses emerging between layers (A. Kelly, High-Strength Materials, Moscow, Mir Publishing House, 1976, pp. 202-206).
The phenomenon of binder destruction in the process of straining takes place because of brittle failure of layers tensioned across fibers, which leads to a substantial reduction in the rigidity of material. Integrity of a composite material deteriorates under loads rather far from breaking ones. Cracking of the binder affects the characteristics of fatigue strength and leads to a number of other undesirable effects, among other things, to a loss of air-tightness. Incorporation of a protective coating increases the product weight (I. F. Obraztsov, V. V. Vasiliev, V. A. Bunakov, Optimal Reinforcement of Composite Shells of Revolution, Moscow, Mashinostroienie Publishing House, 1977, pp. 19-25, 125).
The same shortcomings are mentioned, as a result of shell tests, also in another reference. In loading, the binder soon begins to crack, and there begins a leak of medium because of delaminations (A. Kelly, High-Strength Materials, Moscow, Mir Publishing House, 1976, pp. 202-206).
In high-strength shells (U.S. Pat. No. 3,047,191, U.S. Cl. 220-83, publ. 1962) the number of spirally and annularly wound layers is selected to suit the condition of uniform wall strength by varying the angle of laying the spirals of strips to obtain the predetermined reinforcement structure.
In high-strength composite shells, as shown in French Application No. 2310517, IPC F16L 9/12, publ. 1977 in order to improve interlayer strength, use is made of glass fabric layers.
In U.S. Pat. No. 4,469,138, U.S. Cl. 238-174, publ. 1984 this problem is solved due to incorporation of a modified thermoplastic to ensure better cooperation with reinforcing fibers.
In a known composite shell shaped as a body of revolution as disclosed in U.S. Pat. No. 4,530,379, U.S. Cl. 138-109, publ. 1985, which is made of layers repeating throughout the thickness of its wall and consisting of systems of crisscrossed spiral and annular strips of unidirectional fibers bound by cured polymer binder, the annular strips are arranged at an angle of 80-90.degree. to the shell axis, and the spiral strips, at an angle of 5-75.degree.. As shown by results gained in tests of similar shells, their interlayer strength remains an unsolved problem.
Similar effects of premature destruction are revealed in tests of shells having a rib-cellular structure.
In U.S. Pat. No. 3,083,864, U.S. Cl. 220-83, publ. 1963 composite shells shaped as bodies of revolution formed of layers repeating throughout the thickness of its wall and consisting of systems of crisscrossed spiral and annular strips of unidirectional fibers bound by polymer binders have inadequate strength.
In U.S. Pat. No. 4,284,679, U.S. Cl. 428-218, publ. 1978 and U.S. Pat. No. 4,137,354, U.S. Cl. 428-116, publ. 1979, shells are described which are formed of layers repeating throughout the thickness of its wall and consisting of systems of crisscrossed spiral and annular strips, with stiffening ribs, with outer coating, made respectively of unidirectional fibers bound by a polymer binder, but they have inadequate rigidity.
The shells of a composite material according to USSR Inventor's Certificate No. 763646, Int. Cl. F17C 1/00, publ. 1980 are not provided with adequate solidity, and this reduces their strength.
Methods of forming composite shell are also known in prior art to comprise the steps of impregnating fibers with a binder, laying fiber strands as strips on a mandrel at various angles of inclination to the axis of its rotation so as to form carrying layers, heat treating to cure the binder, and removing the mandrel, as described in U.S. Pat. No. 3,047,191, U.S. Cl. 220-83, publ. 1962, and U.S. Pat. No. 3,083,864, U.S. Cl. 220-83, publ. 1963.
In forming high-strength shells, sealing layers are incorporated beneath the layers wound of reinforcing fibers, as shown in French Patent No. 1,414,309, Int. Cl. 8640, and U.S. Pat. No. 3,303,079, U.S. Cl. 156-72, publ. 1967.
According to U.S. Pat. No. 5,006,291, U.S. Cl. 264-103, publ. 1991, after layers are wound of strips consisting of unidirectional fibers impregnated with a binder, the layers are impregnated additionally.
Japanese Application No. 2-290797, Int. Cl. 8648 1/08, publ. 1990 discloses incorporation of rigid, strong and air-tight rubber which is applied to the mandrel prior to winding layers of strips impregnated with a binder, improving thereby strength and reliability of the product, but its weight is increased.